In the corrugated paperboard industry, long machines comprised of several components are used to make individual pieces of corrugated paperboard from rolls of kraft paper and adhesive. In these machines, a rotary knife is used to cut a continuous web of corrugated paperboard into individual sheets towards the end of the manufacturing process. These individual sheets of corrugated paperboard are then normally stacked and transported for finishing. The quality of the individual sheets and in turn the quality of the end product made from the corrugated sheets depends in large part on the performance of the rotary knife.
In a typical rotary knife, a pair of rotating knife cylinders extend generally transverse across the web. The outer cylindrical wall of each cylinder includes a straight cutting blade that is helically mounted to the respective cylinder. Gears for the cylinders ensure that the cylinders rotate symmetrically so that the blades engage the web and each other to cut the web transversely in a scissors-like fashion. In order to provide a high quality cut, it is important that the blades tightly engage one another when making the scissors cut. A control system for the knife precisely controls the speed of the rotating cylinders so that the forward velocity of the blades during the cut matches the forward velocity of the web through the knife. In addition, the desired length of the individual sheets for the particular order is regulated by controlling the speed of the rotating cylinders when the blades are not engaged, which is in general a different speed than when the blades are engaged to make the cut. This type of variable speed operation is conventional in the art.
Due to the above-described variable speed operation, low inertia rotary knives are used by the assignee of the present application primarily to improve motor drive control and durability. In these low inertia rotary knives, an upper stationary support shaft and a lower stationary support shaft are mounted to the knife frame (normally using retainer mounts). An upper rotatable cylindrical tube is placed concentrically over the upper stationary support shaft and a lower rotatable cylindrical tube is placed concentrically over the lower stationary support shaft. The knife blades are attached to the outer wall of the rotatable cylindrical tubes. Bearings are located between the rotatable cylindrical tubes and the respective stationary support shaft. The rotational inertia of the tubular knife cylinders is significantly less than in systems having solid rotating knife cylinders.
In low inertia rotary knife systems, as well as other rotary knife systems, excessive vibration of the knife cylinders can cause poor quality cuts. Under extreme conditions, the vibrations can sometimes even prevent cuts from occurring altogether. It has been found that cutting frequency has a substantial effect on the amount of steady state vibration, and hence the quality of the cut. The knife cylinder assemblies have certain frequencies at which natural dampening characteristics are insufficient to overcome repeated impulse forces from the engagement of the knife blades. This is especially true when the frequency of the cutting cycle is in sync with the vibration. Vibration problems are more apparent when the knife is operating at high speeds because there is less time for natural damping to occur between cutting cycles. In addition, vibration problems are more apparent on webs that require relatively high cutting forces.